Cracking in Concrete Member

Cracking of concrete is a random process, highly variable and influenced by many factors. Any failure of concrete is the consequence of cracking. It impair the durability of concrete by allowing ingress of aggressive agents. With respect to appearance, cracks are also unacceptable. In addition, cracking may adversely effect the water tightness or sound transmission of structures. Cracking may occur fresh concrete due to plastic shrinkage and plastic settlement. In case of reinforced concrete members, crack generally occur at loads well below service level, and possibly even prior to loading due to restrained shrinkage. Flexural cracking due to loads is not only inevitable, but is actually necessary for the reinforcement to be used effectively.

Influence on Stress-Strain Relationship

It is interesting to note that the two component of concrete, that is, hydrated cement paste and aggregate, when individually subjected to load, exhibit sensibly linear stress-strain relation, although some suggestion about the non-linearity of the stress- strain relation of the hydrated cement paste have been made. The reason for the curved relation in the composite material-concrete- lies in the presence of interfaces between the cement paste and the aggregate and in development of bond microcracks at those interfaces. The progressive development of microcracking was confirmed by neutron radiography.

The development of mocrocracking means that the stored strain energy is transformed into the surface energy of the new crack faces. Because the cracks, develop progressively at interfaces making varying angles with the applied load, and respond the local stress, there is progressive increase in local stress intensity and in the magnitude of the strain. In other words, a consequence of the development of the cracks is a reduction in the effective area resisting the applied load, so that the local stress is larger than the nominal stress based on the total cross-section of the specimen. These changes mean that the strain increases at a faster rate than the nominal applied stress, and so the stress-strain curve continues to bend over, with an apparent pseudo-plastic behavior.

When the applied stress increases beyond approximately 70% of the ultimate strength, mortar cracking (connecting the bond cracks) develops and the stress-strain curve bends over at an increasing rate. The development of a continuous crack system reduces the number of load-carrying paths and, eventually, the ultimate strength of the specimen is reached. This the peak of the stress-strain curve.

In structural design of reinforced concrete, the entire stress-strain curve, often in idealized form, must be considered. For this reason, the behavior of concrete which has a very high strength is of especial interest. Such concrete develops a smaller amount of cracking than normal-strength concrete during all stage of loading; in consequence, the ascending part of the stress-strain curve is steeper and linear up to a very high proportion of the ultimate strength. The descending part of the curve is also very steep so that high strength concrete is more brittle than ordinary concrete, and indeed explosive failure of a local part of specimens of high strength concrete tested in compression has often been encountered. However, the apparent brittleness of high strength concrete is not necessarily reflected in the behavior of reinforced concrete members made with such concrete.

Types of cracking

Non-structural cracks

a) Plastic settlement cracks

Cracking can develop also over obstructions to uniform settlement, e.g., reinforcement or large aggregate particles. This is called plastic settlement.

Details of plastic settlement

Location of appearance

1) In case of over reinforcement member it is generally located in deep section.

2) Where changing in section is inevitable they are seen frequently like waffle slabs.

3) In case of arching they appear at top of column.

Causes

Excess bleeding generally results plastic settlement cracking. Rapid early drying conditions also helps such cracks to be pronounced. Also as stated above over reinforcements and large size aggregate produce differential settlement and finally plastic settlement cracking. Plastic settlement cracking can occur also at normal temperatures but, in hot weather, plastic shrinkage cracking and plastic settlement cracking are sometimes confused with one another.

Arrival time

They are normally appeared within 10 min. to 3 hr of concrete placement.

Remedy

The plastic settlement cracking can be avoided by the use of dry mix, good compaction, and by not allowing too fast build-up of concrete. Reduction of bleeding or revibrating arrest such types of cracking.

b) Plastic shrinkage cracking

When rate of evaporation exceeds the rate at which the bleeding water rise to the surface, plastic shrinkage cracking is likely to occur. Sometimes cracks also form under a layer of water and merely become apparent on drying. The critical evaporation rate is > 1.0 Kg/m2 per hour.

Details of plastic shrinkage cracking

Size of cracks

Plastic shrinkage cracking can be very deep, varying in width between 0.1 and 3 mm, and can be quite short or as long as 1 m.

Encouraging environment

A drop in ambient relative humidity encourage this type of cracking, so that, in fact, the cause of it appear to be rather complex. According to ACI R 305 R-91 the risk of plastic shrinkage cracking is the same at the following combinations of temperature and relative humidity.

410 c (1050 F) and 90 percent.

350 c (950 F) and 70 percent.

240 c (750 F) and 30 percent.

Wind velocity in excess of 4.5 m/s ( 10mph) aggravates the situation.

Location of appearance

The loss of water from by suction by the underlying dry concrete or soil may result cracking. Diagonal cracking is observed in pavements and slabs. Over reinforced section especially steel very near to surface of slab are frequently subjected to such cracking. Random cracking may occur normal reinforced concrete slabs.

Causes

Rapid early drying generally produces this type of cracking. Low rate of bleeding as well can result cracking. As discussed previous, temperature, ambient relative humidity and wind velocity exceeding specified range results crack friendly environment. It should be remembered that evaporation is increased when the temperature of the concrete is much higher than the ambient temperature, under such circumstances, plastic shrinkage can occur even if the relative humidity of the air is high.

Remedy

Complete prevention of evaporation immediately after casting eliminates cracking. The best practice is to protect the concrete from sun and wind, to place and finish fast, and to start curing very soon thereafter. Placing concrete on a dry subgrade should be avoided.

Arrival time

They are generally appeared within 30 min to 6 hr of concrete placement.

c) Corrosion of reinforcement

The corrosion of steel reinforced concrete member by the formation of electro-chemical cell results in cracking (characteristically parallel to the reinforcement), spalling or in delamination of concrete.

This corrosion may occur due to chloride attack and carbonation.

Mechanism of cracking

The corrosion of steel results cracking and further deeper propagation of cracking in two successive steps.

Firstly

The production of corrosion occupies a volume several times larger than the original steel so that their formation results in cracking. This makes it easier for aggressive agents to ingress towards the steel, with a consequent increase in the rate of corrosion.

Secondly

The progress of corrosion at the anode reduce the cross-sectional area of steel, thus reducing its load carrying capacity resulting increase in deflection encouraging cracks to be pronounced.

Location of appearance

These are normally seen in columns and beams where environment is in favor of corrosion.

Cause of cracking

Normally poor quality concrete is subjected to such types of cracking. Inadequate clear cover also makes easy intrusion of aggressive materials like chloride or results carbonation.

The most common reaction in concrete having aggregate with deleterious chemical is reaction between siliceous minerals in the aggregate and alkaline hydroxides in pore water derived from the alkalis (Na2o and k2o) in cement. This reaction can be disruptive and manifest itself as cracking.

Size of cracks

The crack width can range from 0.1 mm to As much as 10 mm in extreme cases. The cracks are rarely more than 25 mm, or at most 50 mm, deep.

Pattern of surface cracking

The pattern of surface cracking induced by the alkali-silica reaction is irregular. Somewhat reminiscent of a huge spider’s web. However, the pattern is not necessarily distinguishable from that caused by sulfate attack or by freezing and thawing, or even by severe plastic shrinkage. Within the concrete, many of the cracks caused by the reaction can be seen to pass through individual aggregate particles but also through the surrounding hydrated cement paste.

Mechanism of cracking

Consequent of the reaction stated above is formation of alkali-silica gel. These gel takes its position in the planes of weakness or pores in the aggregate(where reactive silica is present) or on the surface of the aggregate particles.In the later case, a characteristic altered surface zone is formed. This may destroy the bond between the aggregate and the surrounding hydrated cement paste.

The most common source of Alkalis in concrete is cement. Alkalis become concentrated in some locations, at the expense of others. Such concentration may be caused by moisture gradients or by alternating wetting and drying. The alkalis may also become concentrated by an electric current passed through the concrete which may occur when cathodic protection is used to prevent corrosion of embedded steel.

The additional sources of Alkalis in concrete include sodium chloride present in unwashed sand dredged from the sea or obtained from the desert. Other internal sources of alkalis are some admixtures, especially superplasticizers, or even the mix water. The alkalis from these sources, and also from fly ash and ground granulated blast furnace slag, should be included in the calculation of the amount of Alkalis present.

Location of Appearance

It is normally appeared in damp location.

Remedy

It has been found that expansion due to the Alkali-silica reaction can be reduced and eliminated by the addition to the mix of reactive silica in a finely powdered form. This finely divided siliceous material added to the coarse reactive particles already present would reduce expansion, although the reaction with the alkalis still takes place. These pozzolanic additions, such as crushed pyrex glass or fly ash, have indeed been found effective in reducing the penetration of the coarse aggregate particles. The fly ash should contain no more than 2 to 3 percent by mass of alkalis.

Pozzolanas in the mix are beneficial also because they reduce the permeability of concrete and therefore reduce the mobility of aggressive agents, both those present with in the concrete and those which may ingress. Furthermore, C-S-H formed by pozzolanic activity incorporates a certain amount of alkalis and thus lowers the value of PH. With the decrease in PH alkali-silica reaction rate also decreased.

Silica fume is particularly effective because the silica reacts preferentially with the alkali. Although the product of reaction is the same as that between the alkalis and the reactive silica in the aggregate, the reaction takes place at the very large surface of the fine particles of silica fume. In consequence, the reaction does not result in expansion.

Ground granulated blastfurnace slag is also effective in mitigating or preventing the deleterious effects of the alkali-silica reaction. It should be noted that the presence of ground granulated blastfurnace slag results in a reduced permeability of concrete.

Lithium salts may inhibit also expansion reactions.

Time of Arrival

These cracks normally appeared after 5 years of construction.

e) Crazing

It is an another type of early cracking like plastic shrinkage cracking and plastic settlement cracking which can occur when the surface zone the concrete has a higher water content than deeper in the interior.

Size and pattern of cracking

The pattern of crazing looks like an irregular network with a spacing of up to about 100 mm (4 in). The cracks are very shallow and may not be noticed until etched by dirt; apart from appearance, they are of little importance.

Location of appearance

Walls and slabs normally subjected to crazing cracks.

Causes

Over trowelling and impermeable formwork are primarily results these cracking. Rich mix associated with poor curing also results these cracking.

Remedy

Properly designed concrete with adequate curing reduces such cracking. Poor finishing avoiding over trowelling reduces crazing to great extent.

Time of arrival

They are normally appeared within 1 to 7 days, sometimes crazing may takes much time.

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